It stops short of combining the two airlines' loyalty programs. But it does provide at least some reciprocal benefits between the carriers for the first time since they closed on their merger in 2011.

The new feature is especially noteworthy for Southwest customers, who can now convert Southwest frequent-flier credits into AirTran points that can be used for flights to AirTran's international destinations in Mexico, Bermuda and the Caribbean. And AirTran customers can now convert their frequent-flier credits into points that can be put toward free Southwest flights.

"While the two loyalty programs remain separate for now, the ability to redeem rewards for more destinations represents an enhancement to both programs," Southwest says in a press release announcing the change.

"We are thrilled to share this news today with members of both Rapid Rewards and A+ Rewards," says Kevin Krone, Southwest Airlines vice president of marketing, sales, and distribution, in the release. "Members of both programs have been asking for this ability, and we know they will enjoy using their loyalty rewards to travel to even more destinations."

Members of the carriers' loyalty programs can convert points at the following conversion rates: